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Limnigraf cyfrowy - przykład zastosowania w zlewni Traczówki (Puszcza Niepołomicka, Południowa Polska)

Nycz-Wasilec P., Weiner J.

Czasopismo Techniczne. Środowisko

2002

R. 99, z. 4-Ś

105--111

Water level measurement is an important part of many investigations in natural and technical sciences. Measurements in a well defined watershed allow to record the dynamics of water flow and to estimate water run-off and its components in time. Such investigations require possibly accurate and frequent recording of the water level. Traditional limnigraphs with floats are potentially biased with two kinds of errors: incorrect measurement of the water level resulting from incorrect position of the float, and incorrect reading of a limnigram in the digitalisation process. Precise, pneumatic/pressure limnigraphs with digital data processing and logging are usually expensive and some of them must be used pairwise, because they employ absolute pressure sensors. We constructed and tested in the field the digital limnigraph with a differential pressure sensor. The limnigraph consists of four elements: 1) differential pressure sensor (MPX 5100 DP, Motorola), 2) power supply (6 V lithium battery), 3) adjusting amplifier (AD 627, Analog Devices), 4) datalogger (HOBO H08-002-02, Onset Computer Corporation) (Fig. 1). The range of the measured differential pressure was adjusted to follow the changes in the hydrostatic pressure of 0 - 80 cm water column. Thus, the resolution of the system (0,01 V) reflected the change of 0,32 cm in the water column. The limnigraph was calibrated at the laboratory. The total cost of the device did not exceed 1000 PLN (about 240 USD). The limnigraph was tested in the forest watershed of the Traczówka stream (the Niepołomice Forest, southern Poland) from 02-01-13 till 02-03-11. The device was placed in the limnigraphic well and set for measurement every 15 minutes (Fig. 2). Comparison of the water levels measured at thelevel scale with the water levels calculated from data collected by the limnigraph showed no significant differences between both methods (Tab. 1). Time course of the water level in the Traczówka stream could be explained by both snow melting and rain/snow precipitation (Fig. 3). However, water level measured at the level scale was about 20 cm during the period with extreme low temperature (16-19 January 2002). The discrepancy between the reading and limnigraph data resulted probably from water freezing in the copper measuring tube. Such bias occured probably also from 15 till 17 February 2002. The problem may be helped by adding paraffin to the measuring tube. Nevertheless, the constructed limnigraph proved to be easy in operating, accurate, energy-saving and relatively inexpensive. It can also be applied in long-term, precise water level monitoring of rivers, streams, water basins, as well as ground-waters.